Method for improving the drawing quality of metallic coated ferrous sheet and strip



tates This invention relates to the production of low carbon ferrous metal articles coated with a lower melting point metal such as zinc, aluminum, zinc and aluminum alloys, terne and the like. More specifically, the invention is directed to a method for increasing the ductility of an open annealed and coated sheet or strip so that it can be used in the drawing or forming of metal articles heretofore incapable of being fabricated from such coated stock. Benefit may also be derived from the use of the invention on coated stock which has been box annealed ahead of coating.

The instant invention is primarily applicable to sheet or strip stock coated by the well-known Sendzimir process described in United States Patent 2,110,893. In the preferred practice of the Sendzimir coating process, the cold rolled iron or steel strip is cleaned and prepared for coating by subjecting it to a high temperature oxidation and reduction heat treatment. This heat treatment additionally serves to anneal or normalize the base metal, thereby causing the metal to be recrystallized from the severely strained condition of cold rolling to a substantially softer condition which prevails through the subsequent coating operation. While the resultant coated .product is capable of withstanding relatively severe drawing operations, there are nevertheless instances in which it lacks sufiicient ductility and softeness for deep drawing and the formation of articles having compound curvatures and the like.

While it has hitherto been proposed that the ductility of coated sheets or strip could be improved by subjecting the steel to a box anneal before coating, it has not heretofore been recognized that a second box anneal after coating may be useful in imparting a slight but significant improvement of mechanical properties of the product.

Accordingly, a principal object of the instant invention is the provision of a procedure by means of which iron or steel coated by means of a continuous open annealcoating sequence of operations may have imparted thereto greatly enhanced drawing properties.

An open anneal as referred to herein contemplates any open aneal in which the annealing temperature is above the recrystallization temperature of the ferrous stock and in many cases such temperature may be above the transformation range. neal is that the stock is heated rapidly to temperature and, upon reaching temperature, is cooled at a relatively rapid rate either in air or in a protective atmosphere. Such anneal may be distinguished from a box anneal in that the latter contemplates relatively slow heating to temperature, a prolonged holding at temperature, and subsequent cooling at a relatively slow rate.

Another object of the invention is to improve the drawing quality of coated sheet or strip products produced by procedures involving box annealing of the iron or steel base metal before it enters the coating unit.

Still another object of the instant invention is to produce a deoxidized or killed steel which develops its sta- The essential feature of such an anatent bility against aging after being subjected to an open anneal prior to coating. While killed steel which is subjected to a box anneal as a final processing step possesses non-aging properties, an open anneal or other heat application of short duration prevents the full development of such non-aging properties. It has now been discovered that the non-aging properties of killed steels subjected to short-time heating or cooling may be fully developed by the procedures of the instant invention.

Similarly, it has been found that grades of rimming steel having potential non-aging properties, which have not been developed by reason of a relatively fast heating and cooling cycle, may be stabilized by means of the instant invention.

Briefly stated, the instant invention contemplates subjecting iron or steel coated with zinc, aluminum, zinc and aluminum alloys, terne or the like to a low temperature box anneal following the coating operation and prior to heavy roller leveling, temper rolling or both. While the phenomenon which takes place is not altogether clear, and without being bound by theory, it would appear that a low temperature box anneal following coating is effective in softening the product for two principal reasons: (1) the box annealing causes overaging, and (2) some at least of the thermal and mechanical stresses introduced into the sheet or strip during the coating operation are relieved by the low temperature anneal. I

While the exact cause of overaging has never been satisfactorily established, it is apparent that the hardening effects caused by aging will terminate when all the precipitation hardening elements which are present in supersaturation after an open anneal have been precipitated. It has been observed, for example, that the continued aging of steel at elevated temperatures results in a decrease in hardness. This phenomenon is utilized in the instant invention in that the low temperature anneal is employed to force or synthesize overaging. However, for the anneal to be successful, certain controls must be exercised. Both time and temperature are important, as is the degree of strain of the coated stock at the time of the box anneal. It is also necessary to use a protective atmosphere in the box when the surface of bright coatings must be preserved.

It should be emphasized that the low temperature of the box anneal of the present invention is ofa different order of magnitude from the temperature of the stressrelief anneal or second anneal used to remove residual stresses resulting from rapid cooling during a conventional normalizing operation. A stress-relief anneal is practiced within a temperature range of 950 1200 F., whereas the low temperature box anneal of the present invention may be at a very much lower temperature. It also may be observed that it has heretofore been suggested that low temperature stress relieving at temperatures slightly above room temperature will be useful where the yield strength is not reduced or where it may be increased, but it has not heretofore been realized that a low temperature box anneal as herein taught will result in substantial lowering of hardness and yield and ultimate tensile strengths and a significant increase in tensile elongation. It has been found, for example, that the Rockwell hardness may 'be reduced from 5-10 points, the yield and ultimate strengths lowered 2000-5000 p.s.i., and the tensile elongation increased 25% in a 2-inch gauge length.

For the low temperature box anneal to be eflfective, it has been found that the iron or steel must be held at temperature for a sufficient time to permit the overaging abaaae a 3. phenomenon to take place. Consequently, the preferred temperature for the box anneal must be given in conjunction with the soaking time or time at temperature. For example, little or no benefit will be obtained at a temperature as low as 350 F. unless the soaking time is extended to about 48 hours. It has been found, however, that the time at temperature can be reduced to about 4 hours if the temperature is increased above 400 F. Shorter holding times are effective for many sheets and coils, but it will be understood that steel plant operations must be standardized to allow sufficient time for all material to be treated satisfactorily.

, Where a zinc coating on steel is being produced, the preferred practice is to box anneal at substantially 450 F. for about 24 hours, or at temperatures up to 550 F. to reduce the soaking time to about 8 hours. Where this is done, the Rockwell B hardness of the coated stock has been reduced from 50 to approximately 40, which indicates a marked softening. Temperatures in excess of 550 F. are to be avoided in that the zinc alloys with the iron and the commercial advantages of the product are lessened for many applications. If the zinc has been alloyed with the base metal as an incident of the coating operation, the box annealing temperature is then preferably in the rangeof of 550-750 F.

Where the'ferrous basemetal is coated with aluminum or an. aluminum-silicon alloy, the. preferred temperature is, in the range of 550 F. to 650 F., with a minimum holding time of about 4 hours, in order to obtain uniform properties in all sheets or coils. If the aluminum or the aluminum-coating. is further'alloyed with the iron of the base metal as an incident of, the coatingopera- .tion, the box annealing temperature may be as high as 850 F. without having a deleterious effect on the coating. 'For terne coatings, the preferred annealing cycle is 400; F. to 500 F. fora period of from 4 to 24 hours. WlL le the drawing properties of the base metal would be somwhat improved at higher temperatures, the lead- .tin coatingvwill deteriorate above about 500 F. and consequently care must be'taken to remain within'the prescribed limits. r e

In addition to the specified control of the'box annealing step as such, there is another control which must be exercised if the anneal is to be useful. It has beenfound that if the coated product is given a heavy roller leveling or a temper rolling subsequent to coating and prior to .the low temperature box anneal, the anneal will not produce as soft a product. In this connection, it will be jreadily understood that "dead soft iron or' steel which has been given a high temperature heat treatment as a .final operation is not fiat and is subject to coil breaks, fiuting and stretcher strains when it is cold worked. To .prevent these undesirable effects, it is common practice in the steel industry to roller level and usually temper roll I the" stock. These mechanical operations serve, at least temporarily, to eliminatethe. aforementioned mechanical defects. However, for the coated product to be effectively softened in accordance with the instant invention,

precautions must be taken to prevent over-stressing after coating and before box annealing; and Where more than a light roller leveling is necessary, or where a temper rolling is required, such operations must be performed after the coated metal has been box annealed. In addition, the box anneal must be controlled both as to time However, v

4 relatively rapidly before coating, will not fully develop the non-aging properties of killed stock. The same is true of rimming steels which are potentially non-aging but which, by reason of a shore-time heat treatment, have not had their non-aging properties developed.

Yet when killed or non-aging rimming steels which have been subjected to a short-time heat treatment are subsequently subjected to the low temperature box an neal of the instant invention to enhance their ductility,- their non-aging properties are developed.

The invention having thus been described in several exemplary embodiments, what is desired to besec'ur'ed and protected by Letters Patent is: 4

1. 'A process for improving the drawing qualities of metallic coated ferrous sheet material wherein the metallic coating is chosen from the class consisting of zinc,; aluminium, zinc and aluminium alloys, and terne, said process comprising subjecting the coated sheet material to a low temperature box anneal, the temperature of the box anneal being from 400-850 F., the temperature employed for the chosen coating metal being no greater than that at which the said chosen coating metal will alloy with the ferrous base sheet or form additional alloys not already present on the coated sheet, said low temperature box anneal being conducted'in the absence of temper rolling or heavy roller leveling of the coated sheet material following coating and prior to said low temperature box anneal.

2. The process claimed in claim 1 wherein the metallic coating is Zinc, wherein the box anneal is conducted at atemperature of from 450 to 550 F., and wherein the coated stock is held at temperature for a period of 4 hours up to 24 hours. 3. The process claimed in claim l'wher'ein the metal- .l iccoating comprises-aluminum, wherein the bo'x'anneal is conducted at a temperature of from 550 F to 650 F., and wherein the coated stock is held at temperature for a period of at least 4 hours.

'4. The processclaimed in claim 1 wherein theme'tallic coating is terne, wherein the box anneal is conducted at a temperature of from 400 F. to 500 F. for a period- 'ofat least 4 hours.

5. The process claimed in claim 1 wherein the metallic coating is zinc alloy, and wherein the box anneal is conducted at a temperature of from 550 F. to 750 F. fora period of at least 4 hours.

6. The process claimed in claim 1 wherein the metallic coating comprises aluminum alloy, and wherein the box anneal'is conducted at a temperature of from 550 F. to 850 F. for a period of at least 4 hours.

7 A process for restoring the non-aging properties of normally non-aging ferrous stock which has been subjected to a heat treatment'at 500 F. and higher as an incident of a coating operation, Which heat treatment has at least partially nullified the non-aging properties of said ferrous stock, said process comprising the step of subjecting said stock, subsequent to coating, to a box anneal at a temperature of 400-850 F. for a period of at least 4 hours and not substantially more than 24 hours, the temperature employed for a given coating metal being no greater than that at which said given coating metal will alloy with the ferrous base stock or form additional alloys not already present on the coated stock.

8. A process for improving the drawing qualities of metallic coated sheet material subsequent to the coating:

thereof, said metallic coating being chosen from the class consisting of zinc, aluminium, zinc and aluminium alloys, and terne, which comprises subjecting the coated stock to a box anneal for a soaking period of at least 4 hours and not substantially more than 24 hours at a maximum soaking temperature of substantially 500 F.

for ter ne, 550 F."for z inc,650 F. for'alu mi'niumand aluminium-silicon, 750 F. for m'nc alloys, and 850 F.

5 for aluminium alloys and aluminium-silicon alloys, said box anneal being conducted subsequent to coating but prior to such temper rolling and heavy roller leveling of the coated sheet as required to produce the desired temper and flatness in the coated sheet.

References Cited in the file of this patent UNITED STATES PATENTS 154,435 Adams Aug. 25, 1874 1,745,912 Richardson Feb. 4, 1930 2,067,061 Moffatt Jan. 5, 1937 2,431,947 Martz Dec. 2, 1947 6 FOREIGN PATENTS Australia Sept. 6, 1944 OTHER REFERENCES 5 Rigg: Journal of the Institute of Metals, vol. 54 (1934),

page 183 relied upon.

Schueler: Transactions of the American Electrochemical Society, vol. 47 (1925), page 201 relied upon.

1948 Metals Handbook by the American Society for 10 Metals; pages 704-705.

The Making, Shaping and Treating of Steel, by U.S. Steel, page 666, 7th edition. 

1. A PROCESS FOR IMPROVING THE DRAWING QUALITIES OF METALLIC COATED FERROUS SHEET MATERIAL WHEREIN THE METALLIC COATING IS CHOSEN FROM THE CLASS CONSISTING OF ZINC, ALUMINIUM, ZINC AND ALUMINIUM ALLOYS, AND TERNE, SAID PROCESS COMPRISING SUBJECTING THE COATED SHEET MATERIAL TO A LOW TEMPERATURE BOX ANNEAL, THE TEMPERATURE OF THE BOX ANNEAL BEING FROM 400*-850*F., THE TEMPERATURE EMPLOYED FOR THE CHOSEN COATING METAL BEING NO GREATER THAN THAT AT WHICH THE SAID CHOSEN COATING METAL WILL ALLOY WITH THE FERROUS BASE SHEET OR FORM ADDITIONAL ALLOYS NOT ALREADY PRESENT ON THE COATED SHEET, SAID LOW TEMPERATURE BOX ANNEAL BEING CONDUCTED IN THE ABSENCE OF TEMPER ROLLING OR HEAVY ROLLER LEVELING OF THE COATED SHEET MATERIAL FOLLOWING COATING AND PRIOR TO SAID LOW TEMPERATURE BOX ANNEAL. 